Luminescent image device and combinations thereof with optical filters



July 8, 1969 LUMIESCENT IMAGE DEVICE AND COMBINATIONS THEREOF fil s.LARAcH '3,454,716

WITH OPTICAL FILTERS Filed Jan. 16. 1964 wn ma/4 m- Waff/m1;

VENTOR. i/em/ United States Patent O 3,454,716 LUMINESCENT IMAGE DEVICEAND COMBINA- TIONS THEREOF WITH OPTICAL FILTERS Simon Larach, Princeton,NJ., assignor to RCA Corporation, a corporation of Delaware Filed Jan.16, 1964, Ser. No. 338,191 Int. Cl. H01j 29/89, 29/18; H04n 5/44 U.S.Cl. 1787.86 2 Claims ABSTRACT OF THE DISCLOSURE As used herein: (a) thepeak emission of a phosphor is that maximum light output which, underexcitation, occurs at some specific spectral wavelength as compared tothe lesser light output which occurs at immediately shorter and longerspectral wavelengths; (b) the peak emission intensity of a phosphor isthe amount of light output in an extremely narrow band of wavelengthscentered at the wavelengths of the peak emission which is produced by agiven intensity of excitation; the total emission intensity of aphosphor is the lamount of light output at all wavelengths of thephosphors emission which is produced by a given intensity of excitation.When the emission intensities of two phosphors are compared with eachother, the phosphors are considered to be subjected to equal excitation,and in the same manner at an intensity level within the range of normal,practical operating conditions; (c) the emission band of a phosphor isthe spectral range of wavelengths within which the luminescent emissionis concentrated, the width of the band being equal to the spectral rangeat one-half the peak emission; (d) the transmission band of a filter isthe spectral range of wavelengths within which the transmission of thefilter is concentrated, the width of the band being equal to thespectral transmission range at one-half the peak transmission; (e) thematching of a filter transmission band with a phosphor emission bandmeans that the filter transmission band and the phosphor emission bandare centered at approximately the same spectral wavelength. The filtertransmission band width rnay be narrower than, substantially equal to,or slightly wider than the width of the phosphor emission band; (f) whenone phosphor emission band is described as falling within or outside ofanother phosphor emission band, it is meant that that portion of thespectral band which defines the one phosphors band width is within oroutside that portion of the other spectral band which defines the otherphosphors width; (g) the term standard P1 phosphor means themanganese-activated zinc orthosilicate phosphor identified as No. 21 andstandardized by the U.S, National Bureau of Standards; and (h) the termunaided eye means without the use of an optical filter but does notexclude the use of optical corrective lenses as may be required for aparticular observer.

It is an object of this invention to provide a luminescent image devicelhaving an improved phosphor screen which is especially suited forviewing under high ambient light conditions, for example, in directbright sunlight.

It is also an object of this invention to provide means 3,454,716Patented July 8, 1969 including a luminescent image device whichproduces a high quality image both when viewed directly in low ambientlight and when viewed through a suitable optical filter in high ambientlight.

In accordance with the invention, the phosphor screen of a luminescentimage device includes a first phosphor which has a relatively broademission band and a relatively low peak `emission intensity and a secondphosphor which has a relatively narrow emission band and a relativelyhigh peak emission intensity. The narrow emission band of the secondphosphor lies within the broad emission Iband of the first phosphor. Thetotal emission intensity of the first phosphor is preferably greaterthan that of the second phosphor. In high ambient light, the phosphorscreen is viewed through optical filter means having a narrow (e.g.,less than one hundred angstroms) transmission band approximately matchedto the emission band of the narrow band phosphor, to provide an image ofgood contrast. The filter means is located remote from the phosphorscreen between the ambient light and the observer (c g., as `a pair ofspectacles worn by the observer). In low ambient light, the phosphorscreen is viewed directly without the aid of the filter means to obtainthe benefits of the light output of the first phosphor.

For the purpose of brevity and clarity, the invention is hereinafterdescribed by way of example as involving a cathode ray tube having acathodoluminescent phosphor screen. However, other luminescent imagedevices may instead be used in the practice of the invention, eg., paneldisplay devices having electroluminescent phosphor screens, and displaymeans having X-ray and ultra-violet excited phosphor screens.

It has long been known that an absorption type optical filter can bespectrally matched with, and disposed adjacent to, the phosphor screenof a cathode ray tube to improve the image quality when the tube isviewed under high ambient light conditions. Examples of such knowledgeappear in U.S. Patent 2,419,177 issued to Albert Steadman on Apr. 15,1947, and in U.S. Patent 3,013,114 issued to l. E. Bridges on Dec. l2,1961.

The prior art taught that even in combination with a suitable filter, inorder to get a high ratio of image brilliance to background brilliance,the phosphor of a cathode ray tube screen had to have a broad emissionband so as to produce an image of high brilliance. Thus, to substitute anarrow band phosphor for the prior art broad band phosphor, even thoughthe narrow band phosphor might have a high peak emission intensity,would be contrary to the teaching of the prior art because decrease oftotal light `output was believed to be unacceptable. This belief mayhave been prompted, at least in part, by the fact `that even Where abroad band absorption type faceplate filter was optimumly matched withthe conventional broad-band luminescent emission of a cathode ray tube,the improvement in image contrast was marginal. Such marginalimprovement resulted because the filter, being broad banded to match thephosphor, failed to attenuate a sufficiently large percentage of theambient light which fell upon the phosphor screen. Similarly, tosubstitute a narrow band interference filter for the prior art broadband absorption filter would produce no advantage since this woulddecrease the observable light of the luminescent image as well as theunwanted ambient light. Furthermore, since narrow band interferencefilters refiect rather than absorb the non-transmitted light, the use ofan interference filter as a prior art substitution would increase thereflection of ambient light from the faceplate-filter combination andthus even further degrade the image contrast.

The degree of improvement provided by the present invention over theprior art can be seen by comparing the effectiveness of ambient lightelimination by the present arrangement with that by a typical prior artarrangement in which a broad band absorption filter is matched with abroad band phosphor and positioned adjacent to the phosphor screen.Because the filter in the prior art arrangement is positioned adjacentto the phosphor screen, none of the ambient light directed to theobserver is eliminated. On the other hand, because the filter of thepresent arrangement is positioned at the observer, it eliminatessubstantially all of such light, only that small portion of such lighthaving wavelengths which lie within the narrow transmission band of thefilter not being greatly attenuated.

The ambient light which would, in the absence of any filter, fall on theface of the image device and be reected to the observer may be dividedinto two classes, viz, that having wavelengths which lie within thetransmission band of the filter and that having wavelengths which falloutside the transmission band of the filter. The first of these twoclasses of light is not significantly attenuated by either the prior artarrangement or the present arrangement. However, since the filter of thepresent arrangement has a transmission band, e.g., one-fifth as wide asthe filter of the prior art arrangement (e.g., 100 angstroms as opposedto 500 angstroms), there is only one-fifth as much of this unattenuatedlight which reaches the observer in the present arrangement. Of thesecond of these two classes of light, substantially all thereof iseliminated by the filter of the present arrangement. On the other hand,because the filter of the prior art arrangement is positioned adjacentto the phosphor screen, a significant portion, of this light isreiiected from the front face of the filter toward the observer. Suchlight is the most detrimental of all to image contrast since itincreases the apparent luminous level of the image background.Therefore, the present arrangement eliminates many times the ambientlight that the prior art arrangement eliminates and provides a contrastratio which is many times that provided by the prior art arrangement.

In the drawings:

FIG. 1 is a graph illustrating the spectral energy emissioncharacteristics of a high-peak, narrow-band phosphor suitable for use inthis invention and comparing it with the spectral energy emissioncharacteristics of conventional broadband phosphors; and

FIGS. 2, 3, and 4 are schematic views of different embodiments of theinvention.

Phosphors suitable for use in the practice of this invention preferablyhave a relatively high peak emission intensity. Theoretically, theabsolute value of the peak emission necessary to provide a satisfactoryimage presentation depends upon the brilliance of the ambient orbackground light. If the ambient or background light is of lowbrilliance, then the peak emission intensity of the phosphor can becorrespondingly low and still produce a satisfactory image. Even inbright sunlight ambient, a favorable positioning of the display screenfacing away from direct radiation of the sun lessens the phosphor peakemission intensity requirements. In order to provide satisfactoryoperation in high ambient light, e.g., in bright sunlight, it ispreferred that peak emission intensity of the phosphor be greater thanthe solar radiant power density at the spectral wavelength of thephosphors peak emission. (See, e.g., FIG. 25-1 of IES Lighting Handbook,Illuminating Engineering Society, New York, N.Y.) Under the most adverseambient light conditions where gray-scale image production is desired,the phosphor peak emission intensity is preferably at least twice thesolar radiant power density, or, as compared with standard phosphors,about six or more times the peak emission intensity of the standard P1phosphor. Also theoretically, the emission band of the phosphor need notbe limited to some maximum allowable width. AIf the emission band isgreater than the width of the transmission band of the filter used incombination with the phosphor, the filter will block out the phosphorluminescence which does not fall within the transmission band of thefilter. Some of the emitted energy is then wasted. However, in orderthat a phosphor exhibit the desired peak emission intensity, itordinarily must also have a relatively narrow emission band into whichthe total luminescent energy is concentrated, usually less than aboutangstroms. This situation exists because of a theoretical limit ofmaximum energy conversion efficiency of phosphors in general and becauseof the factors which dictate the practical limits of the amount ofenergy which can be applied to excite the phosphor.

Phosphors having the desired characteristics as set forth abovegenerally contain rare-earth activators, viz, those elements of thePeriodic Table numbered 58 (cerium) through 71 (lutetium). The preferredphosphors are those having a host crystal of a zinc and/ or cadmiumchalcogenide. Such preferred phosphors are hereinafter described indetail in the appendix which follows.

In FIG. l, the curve 10 depicts the response characteristic of acathodoluminescent, thulium-activated zinc sultide phosphor (ZnSzTm),which at 20 C. has an emission band width of approximately 75 angstromsand which peaks at approximately 4,773 angstroms. This phosphor isdescribed in detail as Example 5 in the appendix. Considering thevisible spectrum to be about 3,800 angstroms wide (3,800-7,'600angstroms), this narrow band phosphor has an emission band width whichis less than one fiftieth of the visible spectrum.

The curve 10 of the ZnS:Tm phosphor, is shown superimposed on theresponse characteristic curve 14 of a conventional P4 sulfide phosphorscreen comprising silver activated zinc sulfide and silver activatedzinc-cadmium sulfide such as that described in the booklet RCA Phosphorspublished by Radio Corporation of America in 1961. Such asuperimposition illustrates that the rare-earth activated phosphor has apeak emission whtich is approximately six times the peak emission of theP4 phosphor screen. On the other hand, because of the very narrow bandwidth of the rare-earth activated phosphor, its total luminescence issubstantially less than the total luminescence of the P4 phosphorscreen. Nevertheless, when used in one of the arrangements hereinafterdescribed, this phosphor produces high contrast images in brightsunlight.

Optical lters suitable for use in the practice of the invention shouldhave a relatively narrow transmission band, e.g., less than one-fortiethas wide as the visible spectrum. Interference type filters areespecially desirable because these have narrow band transmissioncharacteristics. Typical filters of the interference type, having atransmission band of 100 angstroms or less, are commercially availablefrom several filter manufacturers. The characteristics of such filtersare described, for example, in Principles of Optics, by Max Barn andEmil Wolf, Pergamon Press Inc., New York, N.Y., 1959.

Cathode ray tubes having different forms of screens which includehigh-peak narrow-band emission phosphors and combinations thereof withdifferent forms of narrowband optical-filter means are described in thefollowing examples.

EXAMPLE A FIG. 2 illustrates a luminescent image device, eg., a cathoderay tube 22, which includes a phosphor screen 24. To present amonochrome image, the phosphor material of the screen 24 may, forexample, comprise blue-emitting ZnS1Tm as described in Example 5 of theappendix and whose response characteristic curve 10 is illustrated inFIG. 1. An observer 26 is provided with a pair of spectacles 28. Thelenses 30 and 32 of the spectacles 28 comprise narrow band opticalfilters. Each of the filter lenses 30 and 32 has a narrow, e.g., 100angstrom wide, transmission band which is centered at the wave length ofthe peak emission of the phosphor screen 24. An ambient light source 34,such as the sun or an incandesent light bulb, illuminates the spacesurrounding the image device 22. The filter lenses 30 and 32, beingpositioned between the observer 26 and the ambient light, serve to blockfrom the observer 26 all of the ambient light except that which fallswithin the very narrow transmission band of the filter lenses 30 and 32.This includes not only that light which is reflected from the face ofthe image device 22, but also that light which is directed from thesource 34 toward the observers eye. As a result of such elimination, theobserver 26 views a readily discernible, high-contrast image on thescreen 24, notwithstanding the lower brilliance of the image due to thenarrow emission band properties of the phosphor screen 24.

Where use in bright sunlight is anticipated, the spectacles 28 may, ifdesired, be such that the narrow band optical filter means matched tothe phosphor emission band is provided as only a part of each lens 30,32, e.g., in a manner similar to a bifocal lens. The remainder of eachlens is provided as a more conventional neutral grey or green absorptiontype filter sun glass lens.

EXAMPLE B FIG. 3 illustrates another luminescent image device, e.g., acathode ray tube 36, which includes a phosphor screen 38. To present amonochrome image, the phosphor of the screen 38 may, for example,comprise the blueemitting ZnSzTm of Example A. An observer 39 is locatedin an enclosure 40 having a window 42 therein through which he mayobserve the phosphor screen 38. The window 42 comprises a narrow bandoptical filter having a transmission band of, e.g., 100 angstroms whichis centered at the wavelength of the peak emission of the phos- -phorscreen 38. Since the observer 39 is separated from the ambient lightsource 34 by the enclosure 40, all of the ambient light except thatlight which falls within the transmission band of the filter window 42is blocked from the observer 39.

EXAMPLE C In accordance with another arrangement the phosphor screen 24(FIG. 2) includes a blue-emitting, narrow band phosphor and ayellow-emitting, broad band phosphor. The blue-emitting phosphor may beZnS:TmLi (Example 7 of the appendix) having an emission band ofapproximately 70 angstroms. The yellow-emitting phosphor may be a(Zn:Cd)S:Ag material having a zinc sulfide to cadmium sulfide ratio ofabout 45/55 and a silver content of about 0.005 Iweight percent andhaving an emission band width of about 1300 angstroms. The emission bandof the narrow band ZnS:'I`mLi phosphor lies outside of, and is spacedfrom, the emission band of the broad band (Zn:Cd)S:Ag phosphor. Theblue-emitting ZnS2TmLi and the yellow-ernitting (Zn:Cd)S:Ag are mixed ina weight ratio of 3/1.

Under relatively low ambient light conditions the observer 26 views thescreen without the use of the filter and sees a black and white image.Under relatively high ambient light conditions, the observer 26 wearsthe spectacles 28, whose lenses have a narrow, e.g., not substantiallygreater than 100 angstroms, transmission band matched to the emissionband of the ZnS:TmLi phosphor and sees a blue monochrome image ofgreatly improved visibility and contrast over that observable withoutthe spectacles 28.

As a variation to this example, a similar arrangement may be used withthe FIG. 3 apparatus wherein the filter window 42 is provided with theproper narrow transmission band.

EXAMPLE D In the arrangement of FIG. 2, the tube 22 may comprise a colorcathode ray tube, such as one having a phosphor screen 24 composed ofthree different color emitting phosphors which are adapted to beselectively excited to produce a color image. The tube 22 may, forexample, comprise: a mosaic dot screen shadow mask type tube such as thecommercially available RCA 21FBP22; a mosaic line-screen feedback tubesuch as that described in U.S. Patent 2,932,756, issued to ArthurLiebscher on Apr. 12, 1960i, or a three layer screen penetration typetube such as that described in U.S. Patent 2,455,710 issued to C. S.Szegho on Dec. 7, 1948. The three phosphors of the color screen 24 maycomprise the narrow band blue-emitting ZnSzTm phosphor (Example 5 of theappendix) and conventional or modified broad-band redemitting andgreen-emitting silver activated zinc-cadmium sulfide phosphors (e.g.,P22 phosphors in RCA Phosphors booklet supra). Each of the filter lenses301 and 32 worn by the observer 26 are made to have a single narrowtransmission band matched to the emission band of the blue-emittingZnSrTm phosphor. Under high ambient light conditions thev observer 26views a blue monochrome image through the filter lenses; under lowamfbient light conditions the observer 26 removes the filter lenses andviews a full-color image.

In a variation of this example, the filter means may be provided asillustrated in FIG. 3. The filter window 42 is made to have a singlenarrow transmission band matched to the blue-emitting ZnSzTm phosphor.

EXAMPLE E In certain applications of cathode ray tubes, a plurality ofdifferent images are simultaneously displayed on a single screen and acorresponding plurality of observers view the screen to visually selecta single image. Such applications, for example, may involve a -trafiiccontrol system wherein three different kinds of information, forexample: (a) aircraft on the runways of an airport; (b) aircraft in aholding pattern aloft adjacent to the airport; and (c) aircraftapproaching the airport from afar, are imaged on a single phosphorscreen. Each of three different men are assigned the job of observingand processing a different one of these images. However, because of thesuperimposition of the images-notwithstanding the fact that they are indifferent colors, e.g., green, yellow, and blue--discrimination by oneobserver of the information for which he is responsible is oftenconfusing and difiicult.

FIG. 4 illustrates an arrangement wherein a plurality of images on asingle screen may be observed by a plurality of observers, each of whomsees only Ithat information for which he is responsible. In FIG. 4, animage display device, e.g., a cathode ray tube 44 includes a phosphorscreen 46 which is composed of three ldifferent narrow-band emissionphosphors, each of which can be separately excited with a differentinformation presentation. For example, the tube 44 may comprise either ashadow mask tube or a line screen feedback type tube as hereinbeforereferred to. The three phosphors of the screen 46, may, for example,comprise green-emitting erbium-activated zinc sulfide (ZnSzEr),yellow-emitting dysprosium-activated zinc sulfide (ZnStDy) andblueemitting thulium-activated zinc sulfide (ZnS:Tm) which aredescribed, respectively, in Examples l, 2, and 5 of the appendix.

A first observer 48 is provided with a pair of spectacles 50 havingoptical filter lenses each of which is made to have a transmission bandmatched to the emission band of the green-emitting ZnSzEr. A secondobserver 52 is provided with a pair of spectacles 54 having opticalfilter lenses each of which is made to have a transmission band matchedto the emission band of the yellow-emitting ZnS:Dy. A third observer 56is provided with a pair of spectacles 58 having optical filter lenseseach of which is made to have a transmission band matched to theemission band of the blue-emitting ZnS:Tm. Thus, each of the observers48, 52, and 56 sees only that image which is displayed in a color whichhis filter spectacles are designed to transmit. Discrimination betweenthat image and the images of the other two colors is substantiallycomplete. This arrangement may find its greater use under conditions oflow ambient light where the spectacles serve to facilitatediscrimination between the different color images. If desired, one ormore of the observers may have filter spectacles that permit him to viewtwo of the different colored images, e.g., by having different filtersin the lenses 30 and 32.

EXAMPLE F In an application wherein a cathode ray tube is to be viewednot only under high ambient light conditions, but also at times underlow ambient light conditions, it may be desirable to retain the highlight output capability of a broad band phosphor and also obtain thehigh contrast advantages of the high-peak, narrow-band phosphors. Toachieve this end, a cathode ray tube may be provided which has aphosphor screen possessing the combined energy response characteristicsof both a broad band phosphor and a narrow band phosphor.

The broad band phosphor has a higher total emission intensity than doesthe narrow band phosphor. The narrow band phosphor, on the other hand,has a higher peak emission intensity than that of the broad bandphosphor, preferably four or more times higher. The emission band of thenarrow band phosphor is also preferably not greater than about one-tenthas wide as, and lies within, the emission band of the broad bandphosphor.

For example, in a monochrome system according to FIG. 2, the phosphorscreen 24 may comprise narrowband blue-emitting ZnSzTm (Example of theappendix) and broad band blue-emitting ZnS:Ag such as that whichconstitutes one of the components of a conventional P4 phosphor mix(e.g., see RCA Phosphors booklet, supra). The response characteristicsof these two phosphors are respectively illustrated in FIG. 1 by curve10 and by the left hand portion 60 of curve 14. In such example, thebroad band phosphor has an emission band of about 750 angstroms(4,300-5,050 angstroms) peaking at about 4,550 angstroms. The narrowband phosphor has an emission band of about 75 angstroms (4,735-4,810angstroms) peaking at about 4,773 angstroms.

The observer 26 is provided with filter lenses 30 and 32, each of whichhas a narrow transmission band matched to the emission band of thenarrow band phosphor ZnSzTm. Under conditions of high ambient light, theobserver 26 views the image presented on the phosphor screen 24 throughthe filter lenses 30 and 32; under low ambient light conditions, theviewer 26 may remove the filter lenses 30 and 32 and view the image onthe screen 24 directly. In either case a blue monochrome image ispresented.

The narrow band phosphor should peak at a Wavelength within the bandwidth of the broad band phosphor. In order that the color of the viewedvimage both with and without use of the filter be substantially the same,it may be preferred that the narrow band phosphor peak at a wavelengthnear the Wavelength at which the broad band phosphor peaks.

The relative proportions of the two phosphors, ZnS1Tm and ZnS:Ag, whichare mixed to provide the screen 24 are dependent upon the particularconditions under which the tube is to be used. If high image contrastunder high ambient light conditions is the primary feature desired, thenthe percentage of the narrow band ZnSzTm phosphor is increased. On theother hand, if considerable use is to be made of the tube under lowambient light conditions without lter spectacles, then a higherpercentage of the broad band ZnSzAg phosphor is used.

As a variation of this example the filter arrangement of Example B maybe used instead of the spectacles.

EXAMPLE G An arrangement involving a mixture of broad band and narrowband phosphors can also be used to present a black and White image underlow ambient light conditions and a monochrome image under high ambientlight conditions. The phosphor screen 24 (FIG. 2) may be made of amixture of phosphors including narrow band blue-emitting ZnSzTm (Example5 of appendix) and broad band blue-emitting ZnSzAg and yellow-emitting(Zn:Cd)S:Ag. The broad band phosphors may be the same as the componentsof a conventional P4 mix (see RCA Phosphors booklet supra), ormodifications thereof, but in slightly different proportions such thatwith the addition of the narrow band ZnSzTm the screen produces asubstantially white light. With such a screen, filter means as disclosedin either Example A or Example B may be used.

Under high ambient light conditions, the observer 26 uses the filterlens spectacles in viewing the screen 24 and sees a blue monochromeimage; under low ambient light conditions he dispenses with the filterlenses and sees a black and white image having the higher brillianceadvantages which the additional light from the broad-band P4 constituentof the phosphor screen provides.

As in a monochrome type screen, the emission band of l the narrow bandphosphor should lie within the emission band of the broad band phosphorof corresponding color.

APPENDIX Some suitable narrow band phosphors may be made by a processwhich comprises reacting a zinc, or cadmium, or zinc-cadmiumchalcogenide with 0.001 to 5.0 mol percent of at least one rare earthelement, as a halide thereof in an oxygen-free ambient, and then coolingthe reaction product. By excluding Voxygen from the ambient during thereaction and by introducing the rare earth element as a halide thereof,this process produces phosphors which exhibits substantial luminescenceemission in relatively narrow spectral bands.

This process applies to phosphors in which the host material is a zinc,or a cadmium, or zinc-cadmium chalcogenide. Chalcogenides, as usedherein, are sulfides, selenides, tellurides, and mixtures thereof. Thepreferred compositions for the host material are those which producesingle phase solid solutions conveniently, although compositions whichproduce more than one phase may also be used, The range in compositionfor the host material may be represented approximately by the molarformula:

aMlSrbMzSezcMaTe where:

M1, M2, and M3 are each at least one member of the group consisting ofzinc and cadmium a=0.0 to 1.0 mol b=0.0 to 1.0 mol c=0.0 to 1.0 mol, anda+b+c=1-00 The preferred host material is zinc sulfide. The alternativehost materials are those in which cadmium is substituted for part or allof the zinc, and/or selenium and/or tellurium is substituted for part orall of the sulfur in the preferred zinc sulfide host material.

At least one rare earth activator is included in the host material inproportions of 0.001 to 5.0 mol percent of the host material. A singlerare earth element is preferred as the activator. Combinations of two ormore rare earth elements may be used. The rare earth elements areselected from the rare earth group of the Periodic Table. The groupconsists of elements numbered 58 (cerium) to 71 (lutetium). Thepreferred rare earth elements are determined by the application in whichthe phosphor is to be used. Because of the nature of the processesdescribed herein, the rare earth element is usually trivalent when it isincorporated in the host material. This is the desired valency for theactivator.

Auxiliary activators may be included with the rare earth activator. Theparticular auxiliary activator which is selected depends upon the usefor the phosphor. In the case of electroluminescent phosphors, it isdesirable to include 0.01 to 1.0 mol percent of copper, as an oxygenfreecompound thereof, in the' host material.

The phosphors of this lprocess are generally prepared in two steps: (1)preparing a batch of the constituents, and then (2) reacting the batchto produce the phosphor. The first step is designed to provide a uniformand intimate mixture of the constituents of the phosphor. The mixture ofconstituents should be as free of oxygen and oxygencotaining compoundsas possible. The constituents may be introduced in various alternativeways. Sulfur, selenium, tellurium, zinc, and cadmium may be introducedin elemental form or as oxygen-free compounds thereof. It is preferredthat the constituents of the host material be prepared first byintimately mixing, as by ball milling chalcogenides of zinc and cadmiumas required, and then calcining the mixture at about 700 to 1400 C. inan oxygenfree atmosphere, preferably hydrogen sulfide. The calcined hostmaterial mixture may be mixed or ground again and recalcined ifnecessary. The rare earth activators and auxiliary activators, ashalides thereof, are then intimately mixed with the prepared batch ofhost material. The activators may be introduced as any halide: finoride,chloride, bromide, and iodide. The batch with the activators therein mayalso be calcined in an oxygen-free atmosphere to remove any volatilematter and to commence the reaction.

If the phosphor is to contain copper, several alternative methods may beused for introducing the copper activator. In one method, the hostmaterial is slurried with a soluble copper halide and then the slurry isthoroughly dried. After drying, the rear earth halide is addedmechanically by any of the above described processes. In a secondmethod, a compound copper-rare earth sulfide is first prepared in thedesired proportion of copper and rare earth. The compound is then mixedwith the host material, and the mixture calcined in the temperaturerange of 800 to 1200 C. in a hydrogen sulfide atmosphere. The mixture isthen reground.

One or more fluxes may -be included in the batch. A suitable flux is amaterial which melts; that is, forms a liquid phase, at temperaturesbelow 800 C. A flux is introduced to lower the reaction temperature, toaccelerate the reaction, and/or to produce a more uniform product. Thepreferred fluxes are alkali halides, such as sodium chloride,sodiumbromide, potassium iodide, lithium chloride, and rubidiumchloride.

The second step is designed to react the host material and activators toform the phosphor without introducing oxygen. To this end, the mixtureof host material and activators is heated in a non-oxidizing oxygen-freeambient at temperatures between 700 and 1400o C. for 0.1 to hours. Inthe preferred process, the batch is heated in a hydrogen sulfideatmosphere for 3 to l8 hours at 900 to 1300 C. 'Ihe optimum heattreatment; that is, the combination of heating time and heatingtemperature, for a particular batch is determined empirically and isdependent in part on the composition of the reaction product. The degreeof heat treatment is generally lower as the content of cadmium,selentium, and tellurium is increased at the expense of zinc and sulfur.A neutral atmosphere or a vacuum may be used instead of a hydrogensulfide atmosphere in both the calcining and reacting steps. Somesuitable gas atmospheres are: argon, neon, nitrogen, ammonia, andmixtures thereof. After the heating is completed, the reaction productis cooled to room temperature and is ready for use as a phosphor. Toimprove homogeneity, the reaction product may be ground and retired oneor more times. If a linx has been used, any excess flux may be removedby leaching.

When excited by 3660 angstrom ultraviolet light, most phosphorsdescribed in the examples below luminesce both at room temperature andat liquid nitrogen temperature (77 K.). The emission is principally innarrow bands, many of which appear to be associated with thecharacteristic 4f-4f transitions of the particular rare 75 10 earthactivator incorporated in the host material. In addition to these narrowbands, there is, in many samples a broad band, either separate or lyingbeneath the narrow bands and dominated by the narrow bands.

Phosphor Examples 8-11 are electroluminescent and are thus especiallysuited for electric field excitation.

Example 1 Mix zinc sulfide with 1.0 mol percent ErCl3. Calcine themixture as described above. Then, heat the calcined mixture at aboutl150 C. for about 1 hour in a hydrogen sulfide atmosphere free ofoxygen, and then cool the reaction product to room temperature. Theproduct is a phosphor having the approximate molar composition ZnS:0.01Er3+ which exhibits a luminescent emission peaked at about 5350angstroms.

Example 2 Mix zinc sulfide with 0.1 mol percent DyF3 and Calcine themixture as described above. Heat the calcined mixture at about l150 C.for about 1 hour in a hydrogen sulfide atmosphere which is free ofoxygen, and then cool the reaction product. The product is a phosphorhaving the molar composition ZnS:0.001 Dy3+ and exhibits a luminescencewhich peaks at about 5750 angstroms.

Example 3 Mix and calcine ZnS with 0.1 mol percent TbF3 as in Example 2.Then heat the calcined mixture for about 3 hours at about 1150 C. in anoxygen-free hydrogen sulfide atmosphere. The product is a phosphorhaving the molar composition ZnS:0.00l Tb3+ and exhibits a luminescencewhich peaks at about 5500 angstroms.

Example 4 Mix and calcine ZnS with 0.1 mol percent HoF3 as in Example 2.Then heat the calcined mixture for about 3 hours at about 1150 C. in anoxygen-free hydrogen sulfide atmosphere. The product is a phosphorhaving the molar composition ZnS:0.001 Ho3+ and exhibits a luminescencewhich peaks at about 4975 angstroms.

Example 5 Mix zinc sulfide with 0.1 mol percent TmF3 and then calcinethe mixture as described above. Heat the calcined mixture at about 1150C. for about 1 hour in a hydrogen sulfide atmosphere free of oxygen, andthen cool the reaction product. The reaction product is a phosphorhaving the approximate molar composition ZnS:0.0-01 Tm3+ and which has aluminescent emission band centered at about 4773 angstroms with a bandwidth of about 50 angstroms. Sharp components of this band may be moreor less noticeable. No color shift has been observed With changes inexcitation level.

Example 6 Mix zinc sulfide with 0.4 weight percent TmF3 and 20 weightpercent NaCl as described above. Heat the calcined mixture at about 1050C. for about 1 hour in an atmosphere of hydrogen sulfide which is freeof oxygen, and then cool the reaction product to room temperature. Thisphosphor has the approximate molar composition ZnS:0.004 Tm3+ and anemission band centered at about 4755 angstroms with a band width ofabout 50 angstroms. Sharp components of this band may be more or lessnoticeable. No color shift has been observed with changes in excitationlevel.

Example 7 Mix zinc sulfide with 0.01 mol percent of thulium chloride and0.01 mol percent of lithium chloride and calcine at C. Heat the calcinedmixture in a hydrogen sulfide atmosphere for one-half hour at 800 C. andthen for one-half hour at 1200 C. The resultant product is a phosphorhaving an emission band which peaks at about 4773 angstroms and which sabout 70 angstroms wide.

1 1 Example 8 Mix 100 grams of pure zinc sulfide with 0.1 gram copper ascuprous chloride and 0.1 gram erbium, as the chloride, and then calcinethe mixture as described above. Heat the calcined mixture at about 1l50C. for about 3 hours in an atmosphere of hydrogen sulfide which is freeof oxygen, and then cool the reaction product to room temperature. Thereaction product has the approximate molar composition ZnS:0.00lCu1+:0.001 Er3+ and exhibits a luminescent emission about 5250 angstromsunder excitation with a 10,000 cycle electric field.

Example 9 Mix 100 grams pure ZnS with 0.1 gram copper as cuprouschloride and 0.1 gram erbium, as the fiuoride, and then calcine themixture. Mix the calcined mixture with 20 weight percent NaCl. Heat theresultant mixture at about 1000 C. for about 1 hour in an atmosphere ofhydrogen sulfide which is free of oxygen, and then cool the reactionproduct to room temperature. The reaction product has the approximatemolar composition ZnS:0.001 Cu1+:0.001 Er3+ and exhibits a luminescentemission in narrow bands peak at about 5300 angstroms when excited witha 10,000 cycle electric field.

Example 10 Mix and calcine ZnS with 0.1 mol percent TbF3 and 0.1 molpercent CuCl as in Example 10. Then heat the calcined mixture for about3 hours at about 1150 C. in an oxygen-free hydrogen sulfide atmosphere.The product is a phosphor having the approximate molar compositionZnS:0.001 Cu1+:0.001Tb3+ and exhibits an electroluminescence which peaksat about 5500 angstroms.

Example 11 said screen including a first phosphor and a second phosphor,said second phosphor having an emission band which is narrower than thatof said first phosphor and which falls within the emission band of saidfirst phosphor and having a peak intensity which is greater than thepeak intensity of said first phosphor, and

(b) optical filter means through which an observer may view said screen,said filter means being disposed between said observer and the ambientlight, said filter means having a narrow transmission band not greaterthan about 100 A. substantially matched to the emission band of saidsecond phosphor.

2. The combination of:

(a) an image device comprising a phosphor screen and rneans for excitingsaid screen to luminescence, said screen including a first phosphorhaving van emission band whose width is not less than 750 angstroms, anda second phosphor having an emission band whose width is not greaterthan angstroms and which lies within the emission band of said firstphosphor, the peak emission intensity of said second phosphor being atleast four times as great as the peak emission intensity of said firstphosphor and the total emission intensity of said iirst phosphor be- Ying greater than that of said second phosphor; and

(b) spectacle means adapted to be worn by an observer and through whichsaid observer may view said screen, said spectacle means comprising anoptical filter lens having a transmission band whose width is notgreater than angstroms and which is substantially centered with respectto the emission band of the second phosphor.

References Cited UNITED STATES PATENTS 2,644,854 7/1953 Sziklai178--7.86` 2,958,002 10/1960 Cusano 313-92 3,250,722 5/ 1960 Borchardt252--301.4 3,271,512 9/1966 Daw 178-5.4

RALPH D. BLAKESLEE, Primary Examiner.

I. A. ORSINO, JR., Assistant Examiner.

U.S. Cl. X.R.

